THIS invention relates to an airborne electromagnetic prospecting system.
There are several airborne electromagnetic prospecting systems, which are typically used to detect underground bodies, such as sulphides, which could contain economic metals, such as copper, zinc and nickel. South African patent no. 98/11489, for example, discloses a prospecting system comprising an aircraft, which is arranged to be towed by a helicopter, and a high drag bird that is connected to the aircraft and towed by the aircraft at an angle of 0approximately 14° below the aircraft.
The aircraft is fitted with a transmitter comprising a transmitter loop and associated electronics for transmitting a primary electromagnetic field, for prospecting the terrain over which the helicopter is flying. A receiver, comprising a three-component receiving coil and associated electronics, is mounted inside the high drag bird for receiving and recording a resulting field, the resulting field having interacted with the underlying terrain. The resulting field comprises a combination of the primary field from the transmitter as well as a secondary field emanating from the underground bodies. The secondary field may then be extracted and processed so as to determine the nature of the underground bodies.
The particular arrangement disclosed in patent no. 98/11489 is advantageous in that the receiver is spaced a sufficiently great distance from the transmitter so as to significantly reduce the primary field component in the resulting field. It is common knowledge that, in order to measure accurately the resulting field, the position of the receiver in the high drag bird relative to the transmitter on the aircraft should ideally remain constant. However, since in the above described arrangement, the aircraft is simply towing the high drag bird containing the receiver, the relative positions tend to vary significantly, with, in particular, variations in the airspeed playing a detrimental role. For example, if the airspeed is reduced, the drag on the high drag bird will also be reduced, and it will therefore fly at an angle greater than 14° relative to the transmitter on the aircraft. This will result in a change in the amplitudes of the primary and secondary fields at the receiver, which adversely effects the accuracy of the recorded data.
Although a small change in the primary field can be effectively compensated for by electronic or signal processing means, a large change can not, because the receiver coil and electronics must have a perfect linear response to signals of different amplitude, which in practice is difficult to achieve. A significant change in the amplitude of the secondary field from the ground results in an incorrect interpretation of the data because the geometry of the transmitter, the receiver and ground surface must be accurately known for interpretation of sub-surface conductors. This follows from the laws of physics dealing with magnetic fields and electromagnetic induction. In addition, if the airspeed falls sufficiently, the receiver bird could drop low enough for it to strike the terrain over which the helicopter is flying, especially if the survey flying height of the aircraft above the ground surface is low.